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Yoon Duk-yong, co-head of the team investigating the sinking of the South Korean warship Cheonan in March, talks next to torpedo parts salvaged from the Yellow Sea. ((Jung Yeon-je/Associated Press))

Evidence overwhelmingly proves North Korea fired a torpedo that sank a South Korean warship in March, killing 46 sailors, investigators said Thursday.

South Korean President Lee Myung-bak vowed "stern action" for the provocation and called an emergency security meeting for Friday, the presidential Blue House said.

"[We] will take resolute countermeasures against North Korea and make it admit its wrongdoings through strong international co-operation," Lee told Australian Prime Minister Kevin Rudd in a phone conversation, the presidential office said.

The White House called the sinking an act of aggression that constitutes a challenge to international peace and security.

Canada's Foreign Affairs Minister Lawrence Cannon said the evidence "points conclusively to a North Korean torpedo having been responsible" and there is "no other plausible explanation."

"Canada strongly condemns this violent act of aggression by the North Korean regime. We are fully supportive of South Korea, our democratic ally and friend," he said in a statement.

But North Korea warned that it will wage "all-out war" if punished for the sinking of the warship. North Korea's official Korean Central News Agency quoted a spokesman from the National Defence Commission as rejecting the findings as a "fabrication."

Sinking killed 46

The long-awaited investigation results, which come from a multinational team, said a torpedo caused a massive underwater explosion that tore the Cheonan apart on March 26.

Navy soldiers stand guard near the wreckage of the naval vessel Cheonan, which was sunk on March 26 near the maritime border with North Korea.

Forty-six sailors died in the frigid Yellow Sea waters near the Koreas' maritime border while 58 were rescued. It was South Korea's worst military disaster since the end of the three-year Korean War in 1953.

Fragments recovered from the area indicate the torpedo came from communist North Korea, investigators said.

Pieces recovered at the sinking site "perfectly match" the schematics of the torpedo included in introductory brochures provided to foreign countries by North Korea for export purposes, chief investigator Yoon Duk-young said.

A serial number on a torpedo fragment also was consistent with markings from a North Korean torpedo that South Korea obtained years earlier, Yoon said.

"The evidence points overwhelmingly to the conclusion that the torpedo was fired by a North Korean submarine," he said in a nationally televised news conference. "There is no other plausible explanation."

Investigators also confirmed that several small North Korean submarines and a mother ship supporting them left a North Korean naval base on the western coast two to three days before the attack and returned to port two to three days after, he said.

All submarines from neighbouring countries were either in or near their respective home bases at the time of the incident, Yoon said.

The incident has raised tensions on the divided Korean peninsula, where the Korean War ended in a truce, rather than a peace treaty.

The investigation team included experts from South Korea, the U.S., Australia, Britain and Sweden.